This invention relates generally to fabricating machine components and more particularly, to methods and apparatus for forming a hardfacing layer on a machine component.
Most known turbine blades are coupled to a central hub that is attached to a driven shaft and the blades are substantially radially disposed with respect to the axis of the hub and shaft. The blades include an airfoil and a high energy, driving fluid impacts the airfoils and imparts a rotational energy that in turn rotates the shaft. Some known combustion turbine blades have shrouds at the outer extremities of the associated airfoils. The blade shrouds are nested in close proximity to each other. Many known turbine blade shrouds have a mechanical interlocking feature in the form of a notch, often referred to a “Z-notch” due to its shape closely resembling the letter Z, that allows each blade to be physically interlocked at its shroud with an adjacent blade.
There are a variety of mechanisms that may cause wear in the region of the Z-notches. For example, during operation of the engine there may be minute, but continuous, vibration of adjacent blades with respect to each other and the hub. The aforementioned interlocking feature facilitates mitigation of airfoil vibration such that the stresses induced within the blades during operation are in turn mitigated. Since the vibration in the blades is mitigated by the close tolerances of the shroud's Z-notches, this condition may increase wear in the vicinity of the shroud's Z-notches as the adjacent notches rub against each other.
Further, during engine starting operations, as the temperatures of the shrouds, airfoils, and hub (as well as all other components that interface with the fluid) vary within each individual component and with respect to other adjacent components, and the engine is accelerated to an operating speed, the blades and shrouds will twist such that the notches will at times contact each other, i.e., attain an interlocked condition. Also, during engine stopping operations there will be a variation in component temperatures substantially reversed from the variations associated with startup as well as an engine deceleration such that the blades and shrouds will twist so that the notches will not contact each other, i.e., attain a non-interlocked condition.
In general, shroud materials do not have the hardness characteristics to withstand the long-term cumulative effects of contact and rubbing. The surface materials of the notches tend to wear. As the notches wear, the effects of the aforementioned twisting and vibration will increase and maintenance shutdowns and repairs may be more frequent. Therefore, a protective material that is compatible with the substrate material and has an increased hardness characteristic, as compared to the substrate materials, to facilitate a decrease in the susceptibility of the notch regions to wear typically is utilized with the Z-notches. This process is often referred to as hardfacing and the associated materials used are referred to as hardfacing materials. The hardface material layers can be formed by welding, spraying or brazing. In general, spray methods may not offer the long-term results achieved by some welding and brazing methods.
Hardfacing using fusion welding methods, including tungsten inert gas (TIG), laser and plasma arc welding methods, have a potential to introduce variables into the hardfacing process that mitigates against repeatability of defect-free layer formation. This situation tends to increase the number and the length of post-weld inspections and weld remediation activities. For example, welding defects typically include weld cracking, porous hardface layers, poor hardface bonding and adhesion, oxidizing of the hardface material and the substrate, and cracking of the substrate due to the creation of a heat affected zone.